In-plane switching mode liquid crystal display device and method of fabricating the same

ABSTRACT

An in-plane switching mode liquid crystal display device includes: first and second substrates facing and spaced apart from each other; a first polarizing plate including a supporting layer, a first polarizing layer and a first protecting layer sequentially on an outer surface of the first substrate; a second polarizing plate including a first compensating layer, a second compensating layer, a second polarizing layer and a second protecting layer sequentially on an outer surface of the second substrate, the first and second compensating layers including positive and negative biaxial retardation films, respectively; and a liquid crystal layer between the first and second substrates.

This application claims the benefit of Korean Patent Application No.2008-0132544 filed on Dec. 23, 2008, which is hereby incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present application relates to a liquid crystal display device, andmore particularly, to an in-plane switching mode liquid crystal displaydevice and a method of fabricating the in-plane switching mode liquidcrystal display device.

2. Discussion of the Related Art

As information age progresses, display devices processing and displayinga large amount of information has been developed. Specifically, flatpanel display (FPD) devices have been required for satisfyingcharacteristics such as light weight, thin profile, and low powerconsumption. As a result, a liquid crystal display (LCD) devices havingadvantages in color reproducibility and profile have been suggested.

The LCD device uses the optical anisotropy and polarization propertiesof liquid crystal molecules to produce an image. Since the liquidcrystal molecules have long thin shapes and a pretilt angle foralignment, the alignment direction of the liquid crystal molecules canbe controlled by changing the pretilt angle due to an applied voltage.Accordingly, the alignment of the liquid crystal molecules changes inaccordance with the alignment direction of the liquid crystal moleculescontrolled by applying a voltage to a liquid crystal layer. As a result,the image is displayed by modulating a polarized light due to theoptical anisotropy of the liquid crystal molecules. Among several typesof LCD devices, active matrix LCD (AM-LCD) devices where thin filmtransistors (TFTs) and pixel electrodes connected to the TFTs aredisposed in matrix are currently widely used because of their highresolution and superior quality for displaying moving pictures.

A related art LCD device includes a color filter substrate having acommon electrode, an array substrate having a pixel electrode, and aliquid crystal layer interposed between the color filter substrate andthe array substrate. In the related art LCD device, the liquid crystallayer is driven by a vertical electric field between the pixel electrodeand the common electrode. The related art LCD device provides a superiortransmittance and a high aperture ratio. However, the related art LCDdevice has a narrow viewing angle because it is driven by the verticalelectric field. To overcome the above disadvantages, various other typesof LCD devices having wide viewing angles, such as in-plane switchingmode (IPS) mode LCD device, have been developed.

FIG. 1A is a cross-sectional view of the related art in-plane switchingmode liquid crystal display device in an ON state, and FIG. 1B is across-sectional view of the related art in-plane switching mode liquidcrystal display device in an OFF state.

In FIGS. 1A and 1B, an in-plane switching (IPS) mode liquid crystaldisplay (LCD) device 10 includes a first substrate 20 having a thin filmtransistor (not shown), a second substrate 30 having a color filterlayer and a black matrix and a liquid crystal layer 40 between the firstand second substrates 20 and 30. A common electrode 22 and a pixelelectrode 24 are alternately formed on the first substrate 20. Anelectric field E is generated according to a voltage applied to thecommon electrode 22 and the pixel electrode 24, and liquid crystalmolecules 40 a and 40 b of the liquid crystal layer 40 are re-alignedalong the electric field E.

In the ON state of FIG. 1A, the voltage is applied to the commonelectrode 22 and the pixel electrode 24, and the electric field E isgenerated. The electric field E has a vertical portion directly over thecommon electrode 22 and the pixel electrode 24, and a horizontal portionbetween the common electrode 22 and the pixel electrode 24. Accordingly,the first liquid crystal molecules 40 a directly over the commonelectrode 22 and the pixel electrode 24 are not re-aligned, and thesecond liquid crystal molecules 40 b between the common electrode 22 andthe pixel electrode 24 are re-aligned along the electric field E. Sincethe liquid crystal layer 40 between the common electrode 22 and thepixel electrode 24 is re-aligned along the horizontal portion of theelectric field E in the ON state, the IPS mode LCD device 10 displaysimages with a wide viewing angle. For example, the images may bedisplayed with a viewing angle of about 80° to about 85° along top,bottom, right and left directions with respect to a normal direction ofthe IPS mode LCD device.

In the OFF state of FIG. 1B, since the voltage is not applied to thecommon electrode 22 and the pixel electrode 24, and the electric field Eis not generated. Accordingly, the first and second liquid crystalmolecules 40 a and 40 b of the liquid crystal layer 40 are notre-aligned.

In the IPS mode LCD device according to the related art, a polarizationstate of the light passing through the IPS mode LCD device is adjustedby first and second polarizing plates on outer surfaces of the first andsecond substrates, respectively.

FIG. 2 is a cross-sectional view showing an in-plane switching modeliquid crystal display device according to the related art.

In FIG. 2, an in-plane switching (IPS) mode liquid crystal display (LCD)device 10 includes first and second substrates 20 and 30 facing andspaced apart from each other, and a liquid crystal layer 40 between thefirst and second substrates 20 and 30. Although not shown in FIG. 2, acommon electrode and a pixel electrode generating an electric field areformed on an inner surface of the first substrate 20.

In addition, first and second polarizing plates 52 and 54 are formed onouter surfaces of the first and second substrates 20 and 30,respectively. The first polarizing plate 52 includes a first supportinglayer 52 a, a first polarizing layer 52 b and a first protecting layer52 c, and the second polarizing plate 54 includes a second supportinglayer 54 a, a second polarizing layer 54 b and a second protecting layer54 c. Each of the first and second supporting layers 52 a and 54 aincludes tri-acetyl cellulose (TAC) having a retardation of about 0.Each of the first and second polarizing layers 52 b and 54 b determininga polarization property is formed by stretching poly-vinyl alcohol (PVA)adsorbing iodine (I) or dye. Each of the first and second protectinglayers 52 c and 54 c also includes TAC.

When the first and second polarizing plates 52 and 54 are disposed suchthat absorption axes of the first and second polarizing plates 52 and 54are perpendicular to each other, the IPS mode LCD device is operated ina normally black mode. Accordingly, when the IPS mode LCD device 10 hasan OFF state, the horizontal electric field is not generated between thecommon electrode and the pixel electrode, and the liquid crystalmolecules of the liquid crystal layer 40 are not re-aligned so that theincident light can penetrate the liquid crystal layer 40 withoutpolarization. As a result, the linearly-polarized light passing throughthe first polarizing plate 52 to have a polarization axis perpendicularto the absorption axis of the first polarizing plate 52 penetrates theliquid crystal layer 40 without change of polarization state, and iscompletely absorbed by the second polarizing plate 54 having anabsorption axis perpendicular to the absorption axis of the firstpolarizing plate 52, thereby a black image displayed.

When the IPS mode LCD device has an OFF state, although the black imageis displayed along a normal direction of the IPS mode LCD device, thebrightness of the black image along up, down, right and left obliquedirections of the IPS mode LCD device may increase due to light leakage.The light leakage is generated because the absorption axes of thepolarizing plates are not perpendicular to each other.

FIGS. 3A and 3B are views showing absorption axes of polarizing platesof an in-plane switching mode liquid crystal display device according tothe related art when viewed at a normal viewing angle and at an obliqueviewing angle, respectively.

In FIG. 3A, when an in-plane switching (IPS) mode liquid crystal display(LCD) device 10 (of FIG. 2) is viewed at a normal viewing angle, a firstabsorption axis ABS1 of a first polarizing plate 52 on an outer surfaceof a first substrate 20 (of FIG. 2) and a second absorption axis ABS2 ofa second polarizing plate 54 on an outer surface of a second substrate30 (of FIG. 2) cross each other with a first angle a1.

In FIG. 3B, however, when the IPS mode LCD device 10 is viewed at anoblique viewing angle, the first absorption axis ABS1 of the firstpolarizing plate 52 and the second absorption axis ABS2 of the secondpolarizing plate 54 cross each other with a second angle a2 greater thanthe first angle a1. Accordingly, the first absorption axis ABS1 of FIG.3B for the obliquely incident light to the IPS mode LCD device 10 iscounterclockwise rotated with respect to the first absorption axis ABS1of FIG. 3A for the normally incident light to the IPS mode LCD device10, and the second absorption axis ABS2 of FIG. 3B for the obliquelyincident light to the IPS mode LCD device 10 is clockwise rotated withrespect to the second absorption axis ABS2 of FIG. 3A for the normallyincident light to the IPS mode LCD device 10.

As a result, the obliquely incident light to the IPS mode LCD device 10passes through the first polarizing plate 52 and is linearly polarizedto have a polarization state having a polarization axis PL perpendicularto the first absorption axis ABS1 of FIG. 3B. However, since thepolarization axis PL is not parallel to the second absorption axis ABS2,the obliquely incident light is not absorbed by the second polarizingplate 54 to cause the light leakage. Therefore, when the IPS mode LCDdevice 10 is viewed along up, down, right and left oblique directions,the luminosity of the black image is deteriorated due to the lightleakage, thereby contrast ratio reduced.

FIG. 4 is a Poincare sphere showing polarization states of obliquelyincident light to an in-plane switching mode liquid crystal displaydevice according to the related art, and FIG. 5 is a view showing abrightness contour line of a black image with respect to a viewing anglein an in-plane switching mode liquid crystal display device according tothe related art.

In FIG. 4, the obliquely incident light to an in-plane switching (IPS)mode liquid crystal display (LCD) device passes through a firstpolarizing plate 52 (of FIG. 2) and is linearly polarized to have apolarization state having a polarization axis PL perpendicular to afirst absorption axis ABS1. Since a second absorption axis ABS2 of asecond polarizing plate 54 (of FIG. 2) is not perpendicular to the firstabsorption axis ABS1, the polarization axis PL is located at differentposition in the Poincare sphere from the second absorption axis ABS2.Accordingly, the obliquely incident light does not display a completeblack image and light leakage occurs.

In FIG. 5, the first polarizing plate 52 and the second polarizing plate54 are disposed such that the first absorption axis ABS1 and the secondabsorption axis ABS2 are parallel to a horizontal direction and avertical direction, respectively. When the IPS mode LCD device displaysa black image, a complete black image without light leakage is viewed ata normal viewing angle having a polar angle θ of about 0° with respectto a z-axis normal to the IPS mode LCD device. However, a light leakageoccurs at an oblique viewing angle along diagonal directions havingazimuthal angles φ of about 45°, 135°, 225° and 315° with respect to anor a y-axis parallel to the IPS mode LCD device. Accordingly, brightnessof the black image increases and contrast ratio is reduced at theoblique viewing angle of the IPS mode LCD device. For example, the blackimage may have relatively high brightness of about 0.018331 (arbitraryunit: A.U.) at an oblique viewing angle having a polar angle of about60° and an azimuthal angle of about 45°. Although a compensation filmhaving a complex retardation is used for the IPS mode LCD device tosolve the above problems, the retardation film causes complication infabrication process and increase in fabrication cost.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an in-plane switchingmode liquid crystal display device and a method of fabricating the samethat substantially obviates one or more of the problems due tolimitations and disadvantages of the related art.

An object of the present invention is to provide an in-plane switchingmode liquid crystal display device where brightness of a black image atan oblique viewing angle is reduced and contrast ratio is improved and amethod of fabricating the in-plane switching mode liquid crystal displaydevice.

Another object of the present invention is to provide an in-planeswitching mode liquid crystal display device where a biaxial film havinga retardation value is used as a supporting layer of a polarizing plateand a method of fabricating the in-plane switching mode liquid crystaldisplay device.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, an in-planeswitching mode liquid crystal display device includes: first and secondsubstrates facing and spaced apart from each other; a first polarizingplate including a supporting layer, a first polarizing layer and a firstprotecting layer sequentially on an outer surface of the firstsubstrate; a second polarizing plate including a first compensatinglayer, a second compensating layer, a second polarizing layer and asecond protecting layer sequentially on an outer surface of the secondsubstrate, the first and second compensating layers including positiveand negative biaxial retardation films, respectively; and a liquidcrystal layer between the first and second substrates.

In another aspect, an in-plane switching mode liquid crystal displaydevice includes: first and second substrates facing and spaced apartfrom each other; a first polarizing plate including a first compensatinglayer, a first polarizing layer and a first protecting layersequentially on an outer surface of the first substrate, the firstcompensating layer including a positive biaxial retardation film; asecond polarizing plate including a second compensating layer, a secondpolarizing layer and a second protecting layer sequentially on an outersurface of the second substrate, the second compensating layer includinga positive biaxial retardation film; and a liquid crystal layer betweenthe first and second substrates.

In another aspect, a method of fabricating an in-plane switching modeliquid crystal display device includes: forming a liquid crystal layerbetween first and second substrates facing and spaced apart from eachother; forming a first polarizing plate including a supporting layer, afirst polarizing layer and a first protecting layer on an outer surfaceof the first substrate; and forming a second polarizing plate includinga first compensating layer, a second compensating layer, a secondpolarizing layer and a second protecting layer on an outer surface ofthe second substrate, the first and second compensating layers includingpositive and negative biaxial retardation films, respectively.

In another aspect, a method of fabricating an in-plane switching modeliquid crystal display device includes: forming a liquid crystal layerbetween first and second substrates facing and spaced apart from eachother; forming a first polarizing plate including a first compensatinglayer, a first polarizing layer and a first protecting layer on an outersurface of the first substrate, the first compensating layer including apositive biaxial retardation film; and forming a second polarizing plateincluding a second compensating layer, a second polarizing layer and asecond protecting layer on an outer surface of the second substrate, thesecond compensating layer including a positive biaxial retardation film.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1A is a cross-sectional view of the related art in-plane switchingmode liquid crystal display device in an ON state;

FIG. 1B is a cross-sectional view of the related art in-plane switchingmode liquid crystal display device in an OFF state;

FIG. 2 is a cross-sectional view showing an in-plane switching modeliquid crystal display device according to the related art;

FIGS. 3A and 3B are views showing absorption axes of polarizing platesof an in-plane switching mode liquid crystal display device according tothe related art when viewed at a normal viewing angle and at an obliqueviewing angle, respectively;

FIG. 4 is a Poincare sphere showing polarization states of obliquelyincident light to an in-plane switching mode liquid crystal displaydevice according to the related art;

FIG. 5 is a view showing a brightness contour line of a black image withrespect to a viewing angle in an in-plane switching mode liquid crystaldisplay device according to the related art;

FIG. 6 is a cross-sectional view showing an in-plane switching modeliquid crystal display device according to a first embodiment of thepresent invention;

FIGS. 7A and 7B are views illustrating positive and negative biaxialretardation films, respectively, for a polarizing plate of an in-planeswitching mode liquid crystal display device according to a firstembodiment of the present invention;

FIG. 8 is an exploded perspective view showing an absorption axis of apolarizing plate and polarization states of incident light passingthrough the polarization plate of an in-plane switching mode liquidcrystal display device according to a first embodiment of the presentinvention;

FIG. 9 is a Poincare sphere showing polarization states of obliquelyincident light to an in-plane switching mode liquid crystal displaydevice according to a first embodiment of the present invention;

FIG. 10 is view showing a brightness contour line of a black image withrespect to a viewing angle in an in-plane switching mode liquid crystaldisplay device according to a first embodiment of the present invention;

FIG. 11 is a view showing brightness distribution at an oblique viewingangle with respect to retardation values of first and secondcompensating layers of an in-plane switching mode liquid crystal displaydevice according to a first embodiment of the present invention;

FIG. 12 is a cross-sectional view showing an in-plane switching modeliquid crystal display device according to a first embodiment of thepresent invention;

FIG. 13 is a Poincare sphere showing polarization states of obliquelyincident light to an in-plane switching mode liquid crystal displaydevice according to a second embodiment of the present invention;

FIG. 14 is view showing a brightness contour line of a black image withrespect to a viewing angle in an in-plane switching mode liquid crystaldisplay device according to a second embodiment of the presentinvention; and

FIG. 15 is a view showing brightness distribution at an oblique viewingangle with respect to retardation values of first and secondcompensating layers of an in-plane switching mode liquid crystal displaydevice according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, similar reference numbers will be used torefer to the same or similar parts.

FIG. 6 is a cross-sectional view showing an in-plane switching modeliquid crystal display device according to a first embodiment of thepresent invention.

In FIG. 6, an in-plane switching (IPS) mode liquid crystal display (LCD)device 110 includes first and second substrates 120 and 130 facing andspaced apart from each other and a liquid crystal layer 140 between thefirst and second substrates 120 and 130.

A gate electrode 121 and a common electrode 122 are formed on an innersurface of the first substrate 120, and a gate insulating layer 123 isformed on the gate electrode 121 and the common electrode 122. An activelayer 124 including an intrinsic semiconductor layer and animpurity-doped semiconductor layer is formed on the gate insulatinglayer 123 over the gate electrode 121, and source and drain electrodes125 and 126 spaced apart from each other are formed on the active layer124. In addition, a pixel electrode 127 connected to the drain electrode126 is formed on the gate insulating layer 123. The pixel electrode 127and the common electrode 122 are disposed to be horizontally spacedapart from each other. The gate electrode 121, the active layer 124, thesource electrode 125 and the drain electrode 126 constitute a thin filmtransistor (TFT) T.

Although not shown in FIG. 6, a gate line and a data line are formedover the inner surface of the first substrate 120, and the TFT T isconnected to the gate line and the data line. Further, a data signal ofthe data line is applied to the pixel electrode 127 according to a gatesignal of the gate line by the TFT T. A passivation layer 128 is formedon the source electrode 125, the drain electrode 126 and the pixelelectrode 127. A liquid crystal layer 140 is formed between thepassivation layer 128 and an inner surface of the second substrate 130.

First and second polarizing plates 152 and 154 are formed on outersurfaces of the first and second substrates 120 and 130, respectively.Although not shown in FIG. 6, adhesive layers may be formed between thefirst substrate 120 and the first polarizing plate 152 and between thesecond substrate 130 and the second polarizing plate 154 for attachment.

The first polarizing plate 152 includes a first supporting layer 152 a,a first polarizing layer 152 b and a first protecting layer 152 c. Thefirst supporting layer 152 a may include tri-acetyl cellulose (TAC)having a retardation of about 0. The first polarizing layer 152 bsubstantially determining a polarization property may be formed bystretching poly-vinyl alcohol (PVA) adsorbing iodine (I) or dye. Inaddition, the first protecting layer 152 c may include TAC.

The second polarizing plate 154 includes a first compensating layer 154a, a second compensating layer 154 b, a second polarizing layer 154 cand a second protecting layer 154 d. The first compensating layer 154 aincludes a positive biaxial retardation film and may be formed bystretching an acrylic resin film such as a polymethyl methacrylate(PMMA) film. The second compensating layer 154 b includes a negativebiaxial retardation film and may be formed by stretching a cyclo-olefinpolymer (COP) film. In addition, the second polarizing layer 154 csubstantially determining a polarization property may be formed bystretching poly-vinyl alcohol (PVA) adsorbing iodine (I) or dye, and thesecond protecting layer 154 d may include TAC.

Hereinafter, the retardation film for the first and second compensatinglayers 154 a and 154 b of the second polarizing plate 154 will beillustrated.

FIGS. 7A and 7B are views illustrating positive and negative biaxialretardation films, respectively, for a polarizing plate of an in-planeswitching mode liquid crystal display device according to a firstembodiment of the present invention.

A retardation film may be classified into uniaxial and biaxial typesaccording to the number of optical axes. In addition, a retardation filmmay be classified into positive and negative types according todifference between refractive indexes along an optical axis directionand along the other direction different than the optical axis direction.For example, a retardation film having one optical axis may beclassified under a uniaxial type and a retardation film having twooptical axes may be classified under a biaxial type. In addition, aretardation film where a refractive index along an optical axisdirection is greater than a refractive index along the other directionmay be classified under a positive type, and a retardation film where arefractive index along an optical axis direction is smaller than arefractive index along the other direction may be classified under anegative type.

A retardation film may be represented by refractive indexes alongdirections in an xyz coordinate. For example, when a retardation film islocated in an xy plane, the x axis and the y axis represent a planedirection of the retardation film, and the retardation film hasrefractive indexes of nx, ny and nz along the x, y and z axes,respectively. In addition, the retardation value of the retardation filmalong the plane direction, i.e., along the x axis or the y axis, isdenoted by Rin, which is defined by (nx−ny) and may be referred to as anplane directional retardation value, and the retardation value of theretardation film along a thickness direction, i.e., along the z axis, isdenoted by Rth, which is defined by (nz−nx) or (nz−ny) and may bereferred to as a thickness directional retardation value. A positiveA-plate and a negative A-plate of a uniaxial retardation film satisfyrelations of (nx>ny=nz) and (nx<ny=nz), respectively. Further, apositive C-plate and a negative C-plate satisfy relations of (nz>nx=ny)and (nz<nx=ny), respectively. Moreover, as shown in FIGS. 7A and 7B, apositive B-plate and a negative B-plate of a uniaxial retardation filmsatisfy relations of (nz>nx>ny) and (nx>ny>nz), respectively.

The first and second compensating layers 154 a and 154 b (of FIG. 6) inthe second polarizing plate 154 of the IPS mode LCD device 110 (of FIG.6) may be formed of the positive and negative B-plates of FIGS. 7A and7B, respectively. In the IPS mode LCD device 110, since the obliquelyincident light passes through the first polarizing plate 152 and thepolarization state of the obliquely incident light is changed by thefirst and second compensating layers 154 a and 154 b of the secondpolarizing plate 154, the obliquely incident light is completelyabsorbed by the second polarizing layer 154 c of the second polarizingplate 154.

FIG. 8 is an exploded perspective view showing an absorption axis of apolarizing plate and polarization states of incident light passingthrough the polarization plate of an in-plane switching mode liquidcrystal display device according to a first embodiment of the presentinvention, FIG. 9 is a Poincare sphere showing polarization states ofobliquely incident light to an in-plane switching mode liquid crystaldisplay device according to a first embodiment of the present invention,and FIG. 10 is view showing a brightness contour line of a black imagewith respect to a viewing angle in an in-plane switching mode liquidcrystal display device according to a first embodiment of the presentinvention.

In FIG. 8, while passing through a first polarizing plate 152 b having afirst absorption axis ABS1, an obliquely incident light of anunpolarized state to an in-plane switching (IPS) mode liquid crystaldisplay (LCD) device 110 (of FIG. 6) is linearly polarized to have afirst polarization state having a first polarization axis PL1 such thata polarization component parallel to the first absorption axis ABS1 isremoved and a polarization component perpendicular to the firstabsorption axis ABS1 remains. In addition, the light of the firstpolarization state is elliptically polarized to have a secondpolarization state having a rotating second polarization axis PL2 whilepassing through a first compensating layer 154 a of a second polarizingplate 154 (of FIG. 6), and the light of the second polarization state islinearly polarized to have a third polarization state having a thirdpolarization axis PL3 while passing through a second compensating layer154 b of the second polarizing plate 154. Since the third polarizationaxis PL3 is parallel to a second absorption axis ABS2 of a secondpolarizing layer 154 c of the second polarizing plate 154, the light ofthe third polarization state is absorbed by the second polarizing plate154. As a result, the obliquely incident light is completely absorbedand the IPS mode LCD device 110 displays a black image. Each of opticalaxes of the first and second compensating layers 154 a and 154 b isparallel to the first absorption axis ABS1 and perpendicular to thesecond absorption axis ABS2.

Although the first absorption axis ABS1 of the first polarizing layer152 b is not perpendicular to the second absorption axis ABS2 of thesecond polarizing layer 152 c at an oblique viewing angle of the IPSmode LCD device 110, the obliquely incident light has the firstpolarization state having the first polarization axis PL1 perpendicularto the first absorption axis ABS1 after passing through the firstpolarizing layer 152 b and has the third polarization state having thethird polarization axis PL3 parallel to the second absorption axis ABS2after passing through the first and second compensating layers 154 a and154 b. Accordingly, the obliquely incident light is completely absorbedby the second polarizing layer 154 c.

The Poincare sphere representing all polarization states of light on asphere has been widely used for designing a compensation film becausepolarization states are easily anticipated by using the Poincare spherewhen an optical axis and a retardation value are given. An equator ofthe Poincare sphere represents a linear polarization. A positive thirdpole S3 and a negative third pole −S3 represent a left handed circularpolarization and a right handed circular polarization, respectively. Inaddition, an upper hemisphere and a lower hemisphere represent a lefthanded elliptical polarization and a right handed ellipticalpolarization, respectively.

In FIG. 9, red, green and blue (R, G and B) obliquely incident lightsare linearly polarized by the first polarizing plate 152 having thefirst absorption axis ABS1 to have the first polarization state havingthe first polarization axis PL1 that is located adjacent to the equatorof the Poincare sphere. In addition, the R, G and B obliquely incidentlights is elliptically polarized by the first compensating layer 152 ato have the second polarization state having the rotating secondpolarization axis PL2 that has the same longitude as the firstpolarization axis PL1 on the Poincare sphere, and is linearly polarizedby the second compensating layer 152 b to have the third polarizationstate having the third polarization axis PL3 that is located adjacent tothe equator of the Poincare sphere. Since the third polarization axisPL3 has substantially the same coordinates as the second absorption axisABS2 of the second polarizing plate 154, the R, G and B obliquelyincident lights may be completely absorbed by the second polarizingplate 154. As a result, the IPS mode LCD device 110 displays the blackimage even with the obliquely incident light, and quality of the blackimage and contrast ratio of the IPS mode LCD device 110 are improved.

In FIG. 10, when the IPS mode LCD device 110 displays a black image, acomplete black image without light leakage is viewed at oblique viewingangles along diagonal directions having azimuthal angles φ of about 45°,135°, 225° and 315° as well as at a normal viewing angle having a polarangle θ of about 0°. For example, the black image may have a brightnessof about 0.000898 (arbitrary unit: A.U.) at an oblique viewing anglehaving a polar angle θ of about 60° and an azimuthal angle φ of about45°. Accordingly, at the oblique viewing angle, the brightness of theblack image may be reduced to about 5% of the brightness of the blackimage, which may be about 0.018331, of the related art IPS mode LCDdevice. Therefore, the brightness of the black image and the contrastratio are improved at the oblique viewing angles of the IPS mode LCDdevice of the first embodiment of the present invention.

The retardation values of the first and second compensating layers 154 aand 154 b may be determined according to the minimum value of thebrightness of the black image at the oblique viewing angles.

FIG. 11 and TABLE 1 are a view and a table, respectively, showingbrightness distribution at an oblique viewing angle with respect toretardation values of first and second compensating layers of anin-plane switching mode liquid crystal display device according to afirst embodiment of the present invention.

TABLE 1 second compensating layer (Rin, nm) 80 90 100 115 125 first 506316 5003 4033 3671 4420 compensating 60 5447 3826 3623 2197 2939 layer70 5265 3494 2180 1613 2347 (Rth, nm) 80 5405 3347 1819 898 1464 90 57283532 1936 1088 1173

In FIG. 11 and TABLE 1, the brightness of the black image is minimizedwhen a retardation value Rth of the positive B-plate of the firstcompensating layer 154 a (of FIG. 6) along a thickness direction (zaxis) is within a range of bout 60 nm to about 100 nm and a retardationvalue Rin of the negative B-plate of the second compensating layer 154 b(of FIG. 6) along a plane direction (x axis or y axis) is within a rangeof about 90 nm to about 130 nm. For example, when the retardation valueRth of the first compensating layer 154 a is within a range of about 70nm to about 90 nm and the retardation value Rin of the secondcompensating layer 154 b is within a range of about 90 nm to about 110nm, the black image may have the minimum brightness.

FIG. 12 is a cross-sectional view showing an in-plane switching modeliquid crystal display device according to a second embodiment of thepresent invention.

In FIG. 12, an in-plane switching (IPS) mode liquid crystal display(LCD) device 210 includes first and second substrates 220 and 230 facingand spaced apart from each other and a liquid crystal layer 240 betweenthe first and second substrates 220 and 230.

A gate electrode 221 and a common electrode 222 are formed on an innersurface of the first substrate 220, and a gate insulating layer 223 isformed on the gate electrode 221 and the common electrode 222. An activelayer 224 including an intrinsic semiconductor layer and animpurity-doped semiconductor layer is formed on the gate insulatinglayer 223 over the gate electrode 221, and source and drain electrodes225 and 226 spaced apart from each other are formed on the active layer224. In addition, a pixel electrode 227 connected to the drain electrode226 is formed on the gate insulating layer 223. The pixel electrode 227and the common electrode 222 are disposed to be horizontally spacedapart from each other. The gate electrode 221, the active layer 224, thesource electrode 225 and the drain electrode 226 constitute a thin filmtransistor (TFT) T.

Although not shown in FIG. 12, a gate line and a data line are formedover the inner surface of the first substrate 220, and the TFT T isconnected to the gate line and the data line. Further, a data signal ofthe data line is applied to the pixel electrode 227 according to a gatesignal of the gate line by the TFT T. A passivation layer 228 is formedon the source electrode 225, the drain electrode 126 and the pixelelectrode 227. A liquid crystal layer 240 is formed between thepassivation layer 228 and an inner surface of the second substrate 230.

First and second polarizing plates 252 and 254 are formed on outersurfaces of the first and second substrates 220 and 230, respectively.Although not shown in FIG. 12, adhesive layers may be formed between thefirst substrate 220 and the first polarizing plate 252 and between thesecond substrate 230 and the second polarizing plate 254 for attachment.

The first polarizing plate 252 includes a first compensating layer 252a, a first polarizing layer 252 b and a first protecting layer 252 c.The first compensating layer 252 a may include a positive biaxialretardation film and may be formed by stretching an acrylic resin filmsuch as a polymethyl methacrylate (PMMA) film. The first polarizinglayer 252 b substantially determining a polarization property may beformed by stretching poly-vinyl alcohol (PVA) adsorbing iodine (I) ordye. In addition, the first protecting layer 252 c may include TAC.

The second polarizing plate 254 includes a second compensating layer 254a, a second polarizing layer 254 b and a second protecting layer 254 c.The second compensating layer 254 a includes a positive biaxialretardation film and may be formed by stretching an acrylic resin filmsuch as a polymethyl methacrylate (PMMA) film. The second polarizinglayer 254 b substantially determining a polarization property may beformed by stretching poly-vinyl alcohol (PVA) adsorbing iodine (I) ordye, and the second protecting layer 254 c may include TAC.

Each of the first and second compensating layers 252 a and 254 a of thefirst and second polarizing plates 252 and 254 may be formed of apositive B-plate satisfying a relation of nz>nx>ny. After the obliquelyincident light to the IPS mode LCD device 210 passes through the firstpolarizing layer 252 b of the first polarizing plate 252, thepolarization state of the obliquely incident light is changed by thefirst and second compensating layers 252 a and 254 a of the first andsecond polarizing plates 252 and 254 to be completely absorbed by thesecond polarizing layer 254 b of the second polarizing plate 254.

FIG. 13 is a Poincare sphere showing polarization states of obliquelyincident light to an in-plane switching mode liquid crystal displaydevice according to a second embodiment of the present invention, andFIG. 14 is view showing a brightness contour line of a black image withrespect to a viewing angle in an in-plane switching mode liquid crystaldisplay device according to a second embodiment of the presentinvention.

In FIG. 13, while red, green and blue (R, G and B) obliquely incidentlights of an unpolarized state pass through the first polarizing layer252 b (of FIG. 12) of the first polarizing plate 252 (of FIG. 12) havingthe first absorption axis ABS1, a component parallel to the firstabsorption axis ABS1 is absorbed and a component perpendicular to thefirst absorption axis ABS1 remains. As a result, the R, G and Bobliquely incident lights have a first polarization state having a firstpolarization axis PL1 that is located adjacent to the equator of thePoincare sphere. In addition, the first polarization state of the R, Gand B obliquely incident lights is changed to an elliptic secondpolarization state having the rotating second polarization axis PL2while passing through the first compensating layer 252 a (of FIG. 12) ofthe first polarizing plate 252, and the second polarization state of theR, G and B obliquely incident lights is changed to an elliptic thirdpolarization state having a rotating third polarization axis PL3 whilepassing through the liquid crystal layer 240 (of FIG. 12). Further, thethird polarization state of the R, G and B obliquely incident lights ischanged to a to an elliptic fourth polarization state having therotating fourth polarization axis PL4 while passing through the secondcompensating layer 254 a (of FIG. 12) of the second polarizing plate 254(of FIG. 12). Since the fourth polarization axis PL4 has substantiallythe same coordinates as the second absorption axis ABS2 of the secondpolarizing layer 254 b, the R, G and B obliquely incident lights may becompletely absorbed by the second polarizing plate 254. As a result, theIPS mode LCD device 210 displays the black image even with the obliquelyincident light, and quality of the black image and contrast ratio of theIPS mode LCD device 210 are improved. An optical axis of the firstcompensating layer 252 a is parallel to the first absorption axis ABS1and perpendicular to the second absorption axis ABS2, and an opticalaxis of the second compensating layer 254 a is perpendicular to thefirst absorption axis ABS1 and parallel to the second absorption axisABS2.

In FIG. 14, when the IPS mode LCD device 210 displays a black image, acomplete black image without light leakage is viewed at oblique viewingangles along diagonal directions having azimuthal angles φ of about 45°,135°, 225° and 315° as well as at a normal viewing angle having a polarangle θ of about 0°. For example, the black image may have a brightnessof about 0.001295 (arbitrary unit: A.U.) at an oblique viewing anglehaving a polar angle θ of about 60° and an azimuthal angle φ of about45°. Accordingly, at the oblique viewing angle, the brightness of theblack image may be reduced to about 7% of the brightness of the blackimage, which may be about 0.018331, of the related art IPS mode LCDdevice. Therefore, the brightness of the black image and the contrastratio are improved at the oblique viewing angles of the IPS mode LCDdevice of the second embodiment of the present invention.

The retardation values of the first and second compensating layers 252 aand 254 a may be determined according to the minimum value of thebrightness of the black image at the oblique viewing angles.

FIG. 15 and TABLE 2 are a view and a table, respectively, showingbrightness distribution at an oblique viewing angle with respect toretardation values of first and second compensating layers of anin-plane switching mode liquid crystal display device according to asecond embodiment of the present invention.

TABLE 2 second compensating layer (Rin, nm) 160 180 200 220 240 first 654283 3284 3245 4181 6059 compensating 75 4213 2691 2234 2878 4607 layer85 5523 2866 1447 1349 3026 (Rin, nm) 95 6886 3629 1829 1295 2958 1058702 4876 2756 1760 3137

In FIG. 15 and TABLE 2, the brightness of the black image is minimizedwhen a retardation value Rin of the positive B-plate of the firstcompensating layer 252 a (of FIG. 12) along a plane direction (x axis ory axis) is within a range of bout 60 nm to about 110nm and a retardationvalue Rin of the positive B-plate of the second compensating layer 254 a(of FIG. 12) along a plane direction (x axis or y axis) is within arange of about 170 nm to about 240 nm. For example, when the retardationvalue Rin of the first compensating layer 252 a is within a range ofabout 80 nm to about 100 nm and the retardation value Rin of the secondcompensating layer 254 b is within a range of about 190 nm to about 220nm, the black image may have the minimum brightness.

Consequently, in the IPS mode LCD device according to the presentinvention, since each of the first and second polarizing plates includesa biaxial retardation film, the obliquely incident light is compensatedand the light leakage at the oblique viewing angle is prevented. As aresult, the brightness of the black image at the oblique viewing angleis reduced, and the contrast ratio of the IPS mode LCD device at theoblique viewing angle is improved, thereby the display quality improved.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the IPS mode LCD device andthe method of fabricating the IPS mode LCD device of the presentinvention without departing from the spirit or scope of the invention.Thus, it is intended that the present invention cover the modificationsand variations of this invention provided they come within the scope ofthe appended claims and their equivalents.

1. An in-plane switching mode liquid crystal display device, comprising:first and second substrates facing and spaced apart from each other; afirst polarizing plate including a supporting layer, a first polarizinglayer and a first protecting layer sequentially on an outer surface ofthe first substrate; a second polarizing plate including a firstcompensating layer, a second compensating layer, a second polarizinglayer and a second protecting layer sequentially on an outer surface ofthe second substrate, the first and second compensating layers includingpositive and negative biaxial retardation films, respectively; and aliquid crystal layer between the first and second substrates.
 2. Thedevice according to claim 1, wherein the first compensating layer has athickness directional retardation value (Rth) within a range of bout 60nm to about 100 nm and the second compensating layer has a planedirectional retardation value (Rin) within a range of about 90 nm toabout 130 nm.
 3. The device according to claim 1, wherein the firstcompensating layer includes a stretched polymethyl methacrylate (PMMA)film and the second compensating layer includes a stretched cyclo-olefinpolymer (COP) film.
 4. The device according to claim 1, wherein thesupporting layer includes tri-acetyl cellulose (TAC) having aretardation of about 0, wherein each of the first and second polarizinglayers includes a stretched poly-vinyl alcohol (PVA) adsorbing one ofiodine (I) and dye, and wherein each of the first and second protectinglayers includes tri-acetyl cellulose (TAC).
 5. The device according toclaim 1, wherein absorption axes of the first and second polarizingplates are perpendicular to each other, and wherein each of optical axesof the first and second compensating layers is parallel to theabsorption axis of the first polarizing layer and perpendicular to theabsorption axis of the second polarizing layer.
 6. An in-plane switchingmode liquid crystal display device, comprising: first and secondsubstrates facing and spaced apart from each other; a first polarizingplate including a first compensating layer, a first polarizing layer anda first protecting layer sequentially on an outer surface of the firstsubstrate, the first compensating layer including a positive biaxialretardation film; a second polarizing plate including a secondcompensating layer, a second polarizing layer and a second protectinglayer sequentially on an outer surface of the second substrate, thesecond compensating layer including a positive biaxial retardation film;and a liquid crystal layer between the first and second substrates. 7.The device according to claim 6, wherein the first compensating layerhas a plane directional retardation value (Rin) within a range of bout60 nm to about 110 nm and the second compensating layer has a planedirectional retardation value (Rin) within a range of about 170 nm toabout 240 nm.
 8. The device according to claim 6, wherein each of thefirst and second compensating layers includes a stretched polymethylmethacrylate (PMMA) film.
 9. The device according to claim 6, whereineach of the first and second polarizing layers includes a stretchedpoly-vinyl alcohol (PVA) adsorbing one of iodine (I) and dye, andwherein each of the first and second protecting layers includestri-acetyl cellulose (TAC).
 10. The device according to claim 6, whereinabsorption axes of the first and second polarizing plates areperpendicular to each other, wherein an optical axis of the firstcompensating layer is parallel to the absorption axis of the firstpolarizing layer and perpendicular to the absorption axis of the secondpolarizing layer, and wherein an optical axis of the second compensatinglayer is perpendicular to the absorption axis of the first polarizinglayer and parallel to the absorption axis of the second polarizinglayer.
 11. A method of fabricating an in-plane switching mode liquidcrystal display device, comprising: forming a liquid crystal layerbetween first and second substrates facing and spaced apart from eachother; forming a first polarizing plate including a supporting layer, afirst polarizing layer and a first protecting layer on an outer surfaceof the first substrate; and forming a second polarizing plate includinga first compensating layer, a second compensating layer, a secondpolarizing layer and a second protecting layer on an outer surface ofthe second substrate, the first and second compensating layers includingpositive and negative biaxial retardation films, respectively.
 12. Themethod according to claim 11, wherein the first compensating layer has athickness directional retardation value (Rth) within a range of bout 60nm to about 100 nm and the second compensating layer has a planedirectional retardation value (Rin) within a range of about 90 nm toabout 130 nm.
 13. The method according to claim 11, wherein the firstcompensating layer is formed by stretching a polymethyl methacrylate(PMMA) film and the second compensating layer is formed by stretching acyclo-olefin polymer (COP) film.
 14. The method according to claim 1,wherein the supporting layer includes tri-acetyl cellulose (TAC) havinga retardation of about 0, wherein each of the first and secondpolarizing layers is formed by stretching a poly-vinyl alcohol (PVA)adsorbing one of iodine (I) and dye, and wherein each of the first andsecond protecting layers includes tri-acetyl cellulose (TAC).
 15. Themethod according to claim 1, wherein absorption axes of the first andsecond polarizing plates are perpendicular to each other, and whereineach of optical axes of the first and second compensating layers isparallel to the absorption axis of the first polarizing layer andperpendicular to the absorption axis of the second polarizing layer. 16.A method of fabricating an in-plane switching mode liquid crystaldisplay device, comprising: forming a liquid crystal layer between firstand second substrates facing and spaced apart from each other; forming afirst polarizing plate including a first compensating layer, a firstpolarizing layer and a first protecting layer on an outer surface of thefirst substrate, the first compensating layer including a positivebiaxial retardation film; and forming a second polarizing plateincluding a second compensating layer, a second polarizing layer and asecond protecting layer on an outer surface of the second substrate, thesecond compensating layer including a positive biaxial retardation film.17. The method according to claim 16, wherein the first compensatinglayer has a plane directional retardation value (Rin) within a range ofbout 60 nm to about 110 nm and the second compensating layer has a planedirectional retardation value (Rin) within a range of about 170 nm toabout 240 nm.
 18. The method according to claim 16, wherein each of thefirst and second compensating layers is formed by stretching apolymethyl methacrylate (PMMA) film.
 19. The method according to claim16, wherein each of the first and second polarizing layers is formed bystretching a poly-vinyl alcohol (PVA) adsorbing one of iodine (I) anddye, and wherein each of the first and second protecting layers includestri-acetyl cellulose (TAC).
 20. The method according to claim 16,wherein absorption axes of the first and second polarizing plates areperpendicular to each other, wherein an optical axis of the firstcompensating layer is parallel to the absorption axis of the firstpolarizing layer and perpendicular to the absorption axis of the secondpolarizing layer, and wherein an optical axis of the second compensatinglayer is perpendicular to the absorption axis of the first polarizinglayer and parallel to the absorption axis of the second polarizinglayer.